Drillrods for use in oil and natural gas exploration are manufactured according to the API SPEC 5DP standards. The structure thereof has an externally threaded drillrod coupler and an internally threaded drillrod coupler which are respectively frictionally butt-welded at the two ends of the drillrod tube body. Drillrods in compliance with the API SPEC 5DP standards are of a low alloy steel material.
With the development of the oil industry, the conditions in which drillrods operate become more and more severe, drillrods of the low alloy steel material as per the API SPEC 5DP standards now fail to fulfill the increasingly harsh requirements of well drilling operation, and there exists an urgent need for a high alloy drillrod. Therefore, manufacturers abroad have developed aluminum alloy drillrods and titanium alloy drillrods.
The aluminum alloy drillrods are manufactured as per the ISO 15546 standards. The aluminum alloy drillrod is formed from an aluminum alloy drillrod tube body connected by means of fine threads with an externally threaded coupler made of low alloy steel and an internally threaded coupler made of low alloy steel.
The structure of the titanium alloy drillrod is similar to that of the aluminum alloy drillrod. U.S. Pat. No. 6,305,723 proposes a method for connecting the titanium alloy drillrod coupler, which is also carried out by connecting the titanium alloy drillrod tube body by means of fine threads with an externally threaded coupler made of low alloy steel and an internally threaded coupler made of low alloy steel.
The utilization of the aluminum alloy drillrod and the titanium alloy drillrod has two major objectives as follows: one is to drill a super deep well by taking advantage of the low specific gravity property of the aluminum alloy drillrod and the titanium alloy drillrod, and the other is to drill a sulfur-containing well by taking advantage of the resistance property of the aluminum alloy drillrod and the titanium alloy drillrod to stress corrosion by sulfides.
However, for some CO2-containing gas fields whose stratum is of compact sandstone, in the case of a conventional method of operation which employs a drillrod for drilling a well and an oil tube for completing the well (i.e. a method in which a drillrod is used in a well-drilling liquid to drill the well, and after finishing the well drilling, the drillrod is lifted out, and an oil tube is lowered therein for completing the well), due to the contamination of the well-drilling liquid to the storage layer, the yield is only tens of thousands of cubic meters/day; in addition, due to a relatively high level of CO2, superior 13Cr high alloy oil tube products that are costly must be used, resulting in an extremely high cost, meaning low value in industrial exploration. If a nitrogen well-drilling process can be employed, there is no contamination of the well-drilling liquid to the storage layer during well drilling, and a high yield of millions of cubic meters of natural gas per day can be achieved. However, when the nitrogen well-drilling process is used, the drillrod cannot be lifted out to exchange into the oil tube for well completion (because a well blowout will occur if the drillrod is lifted out in nitrogen, which may further lead to a tragic event with the well destroyed and peopled killed, if the drillrod is required to be lifted out, the nitrogen has to be exchanged into a well-drilling liquid), otherwise the production layer would be contaminated, lowering the yield back to tens of thousands of cubic meters/day and losing the positive effects of the nitrogen well-drilling.